In order to decrease the capacitance of metal interconnects, silicon dioxide (SiO2) can be replaced by a low dielectric constant (low-k) materials as an interlevel dielectric (ILD) layer. As used herein a low-k material has a dielectric constant less than that of SiO2 (i.e., less than approximately 4.0.) One desirable low-k material is a low-pressure gas or vacuum, which are both commonly termed air or air gaps.
Air gaps can be formed by placing two metal interconnects in close proximity to each other so that when an insulating material is deposited over the surface of a semiconductor wafer, the insulating material will not completely fill the space between the two metal interconnects and hence, an air gap is formed in the space. However, even when an air gap is formed, there exists some capacitance coupling between the two metal interconnects due to their proximity to each other. Since increasing the spacing between the two metal interconnects decreases capacitance coupling more than forming air gaps, design rules often require that the metal interconnects that are sensitive to capacitance coupling are often spaced apart a distance that is too large to form air gaps. However, this undesirable increases the size of the semiconductor device or die. A less desirable low-k material than air (i.e., a material having a higher dielectric constant than air) is deposited between the metal interconnects that are sensitive to capacitance. It would be extremely desirable to further decrease capacitance between the metal lines.
There exists many approaches to forming air gaps. Many try to prevent subsequently formed misaligned vias from puncturing an underlying air gap. When filling the via, the chemicals may react with the dielectric surrounding the air gap and lead to a poisioned via. One approach includes forming a hard mask, while another approach includes forming a landing pad. Both processes undesirably require additional patterning steps. Furthermore, the hard mask is a high dielectric constant material and therefore undesirably increases the overall dielectric constant between metal interconnects. Thus, a need exists for a process that allows for via misalignment when forming air gaps, especially, between metal interconnects that are spaced too far apart to traditionally have air gaps formed there between.
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